A conventional RF heating system for use in a microwave oven includes a high power direct oscillator device called “magnetron”, which uses a vacuum tube, and an antenna (or radiator) for radiating the electromagnetic wave that is generated by the magnetron inside the heating chamber. The frequencies of electromagnetic waves for such RF heating usually fall within an ISM band, and the oscillation frequency of the magnetron is ordinarily defined to be a predetermined value falling within the range of 2.40 GHz through 2.50 GHz. Actually, however, the oscillation frequency of the magnetron will fluctuate according to the voltage applied to the magnetron and the impedance inside the heating chamber. As a result, the spectrum of its oscillation will eventually cover almost the entire 100 MHz range from 2.40 GHz through 2.50 GHz.
To overcome such a problem, a solid-state RF heating system, including an oscillator and a solid-state power amplifier in place of a magnetron, has been researched and developed. Such a system is now proposed because an RF semiconductor device of GaN or SiC (which will be referred to herein as a “semiconductor power amplifier”) has become more and more popular these days. An RF heating system that uses such a semiconductor power amplifier receives an RF signal, supplied from an oscillator, amplified by the semiconductor power amplifier and has the electromagnetic waves radiated from a radiator into the heating chamber with a lot of power.
A solid-state RF heating system can radiate electromagnetic waves with a line spectrum and with almost no noise components. In addition, by adjusting the settings of its oscillator, the radiation frequency of the line spectrum can be varied arbitrarily within the range of 2.40 GHz to 2.50 GHz.
However, a semiconductor power amplifier would be easily damaged under the heat when exposed to an intense reflected wave, which is a serious problem that must be solved to actually use it in that field of applications. If such a semiconductor power amplifier is used in the field of telecommunications, electromagnetic waves are radiated into a free space, and therefore, there is little need to keep the semiconductor power amplifier from getting damaged by reflected waves. On the other hand, if such a semiconductor power amplifier is used in a closed environment where intense electromagnetic waves are radiated into the heating chamber of a microwave oven, for example, strong reflected waves are easily produced inside the heating chamber. For that reason, the semiconductor power amplifier must be shielded from such reflected waves in one way or another.
Patent Document No. 1 discloses an example of an RF heating system with such a semiconductor power amplifier. As shown in FIGS. 9(a) and 9(c), such an RF heating system performs a monitor mode operation for measuring the intensity of a reflected wave with the intensity of a radiated wave (i.e., radiation power) kept low and with the frequency sweeping continuously right after the system has been turned ON. In the example illustrated in FIG. 9(c), the frequency is continuously varied in the monitor mode from 2.40 GHz through 2.50 GHz. When the intensity of the reflected wave produced in such a situation is sensed, it can be seen that the intensity of the reflected wave varies significantly according to the frequency of the electromagnetic wave as shown in FIG. 9(b).
By performing such a monitor mode operation, a frequency that will lead to the lowest reflected wave intensity can be determined. After the oscillation frequency has been fixed at such a frequency that has been determined in this manner, the output (i.e., the radiated wave) is raised as shown in FIG. 9(a), thereby starting radiation for the purpose of heating. In the system disclosed in Patent Document No. 1, if the intensity of the reflected wave becomes equal to or higher than a predetermined value for some reason during the heating process, the output (i.e., the electromagnetic waves radiated) is decreased and the heating process is stopped in order to prevent the solid-state amplifier from getting damaged by the reflected wave.
Such a solid-state RF heating system not only can perform an RF heating process at such a frequency at which the radiated waves are hardly reflected and the object can be heated with high absorption efficiency but also can keep the power amplifier from getting damaged by the reflected waves.
On the other hand, Patent Document No. 2 discloses an RF heating system that detects impedance in the heating chamber and controls the oscillation frequency based on the result of the detection. By adjusting the oscillation frequency, the system tries to get impedance matching always done and get the object cooked evenly.
Meanwhile, Patent Document No. 3 discloses a microwave processor that memorizes the relation between the reflection power and the frequency by making the microwave frequency sweep in the range of 2.4 MHz through 2.5 MHz and by detecting the reflected current. Such a microwave processor extracts a frequency that will lead to the lowest reflection power as a heating frequency by reference to the memorized relation between the reflection power and the frequency.